Barash, a professor of psychology and biology at the University of Washington, has written a highly enjoyable account of things humans have yet to learn about themselves: “known unknowns” he calls them, quoting former secretary of defense Donald Rumsfeld. The evolutionary reasons behind such human characteristics as homosexuality, concealed ovulation, female orgasm, play, and social bonding still perplex scientists, and Barash shares several of the leading theories behind each one. Concealed ovulation, for example, may allow women to exercise greater control over their choice of a sexual partner.

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This article was originally published with the title "Homo Mysterious: Evolutionary Puzzles of Human Nature."